To reproduce:

1. Open attached project.
2. Execute menu item: BUG/Open Editor Window
3. Press anywhere on newly opened custom editor window.

Expected result: No error.
Actual: Null reference exception in DoPlanarHandle:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Handles.DoPlanarHandle (PlaneHandle planeID, Vector3 position, Quaternion rotation, Single handleSize) (at C:/buildslave/unity/build/Editor/Mono/EditorHandles/PositionHandle.cs:147)
UnityEditor.Handles.DoPositionHandle_Internal (Vector3 position, Quaternion rotation) (at C:/buildslave/unity/build/Editor/Mono/EditorHandles/PositionHandle.cs:90)
UnityEditor.Handles.DoPositionHandle (Vector3 position, Quaternion rotation) (at C:/buildslave/unity/build/Editor/Mono/EditorHandles/PositionHandle.cs:61)

Notes: The handle is actually drawn when you pass camera to it. I know it's not quite intended usage of this function, but other Handle methods seem to work and render fine on editor windows. If DoPositionHandle should not work, maybe we should throw InvalidOperationException with message which describes that this should not be used in editor windows.

Reproduced on: 5.6.0.b5, 5.5.0p1, 5.4.1p2